Did you try using 16 bit depth (instead of the default 8)? PNG also lets you choose the compression level - default is 15% - maybe try turning to 0?
– Christopher BennettOct 31 '21 at 13:37
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Related: https://blender.stackexchange.com/questions/239795/why-is-my-saved-png-not-as-good-as-the-render/239804#239804
– josh sanfeliciOct 31 '21 at 14:58
It's on a different background, make sure you use the same BG twice to rule out optical illusions. Also make sure you checked RGBA in the image format.
– Frederik SteinmetzNov 01 '21 at 13:35